296 MEMOIRS OF THE CARNEGIE MUSEUM 
Truncilla triquetra Rafinesque; T. perplexa rangiana (Lea); Micromya fabalis 
(Lea); Lampsilis ventricosa (Barnes); L. ovata (Say); L. cariosa (Say); L. mul- 
tiradiata (Lea); L. radiata (Gmelin); L. luteola (Lamarck); L. ligamentina (La- 
marck); L. orbiculata (Hildreth); L. recta (Lamarck); L. nasuta (Say); L. iris 
(Lea); ZL. parva (Barnes); L. alata (Say); L. gracilis (Barnes); Obovaria retusa 
(Lamarck); O. circulus (Lea); O. ellipsis (Lea); Plagiola securis (Lea); Obliquaria 
refleca Rafinesque; Ptychobranchus phaseolus (Hildreth). 
This type resembles the one just described in so far as the two lamine of 
the gill yield at the edge to the tendency of the marsupium to swell. The swelling 
of the gill itself is generally not carried to the same excessive degree as in the 
Anodonta-group, although it is sometimes quite considerable; in its stead, the chief 
expansion takes place at the edge itself, and in this case beyond it. The thickened 
tissue is not simply stretched out in a transverse direction, but it bulges out, 
thus protruding beyond the original edge of the gill (See Pl. LX XXVIII, fig. 14). 
The latter is always clearly indicated by a line at which the filaments and their 
chitinous rods stop; and this line may be seen with the naked eye or with a low 
power lens. Beyond this line the gill sometimes projects to a considerable distance, 
and this projection, into which the septa are continued, acquires a peculiar appear- 
ance. It does not project and bulge out uniformly, but the septa on the inside, 
acting as stays, check the bulging at their lines of insertion, and thus the edge of 
the marsupium assumes a beaded appearance, each bead representing a water-tube, 
being separated from the adjoining bead by a kind of constriction or sulcus, repre- 
senting the septum. This protruding, beaded structure of the marsupial edge is 
very characteristic of these forms, and is distinctly visible to the naked eye. 
Apparently Simpson’s diagnostic character, that here the ovisaes are distinctly 
“marked out by a sulcus,” refers to the edge of the marsupium; for upon the 
lateral faces of the marsupium there are no such distinet sulci, and there is hardly 
any difference in the appearance of these faces from what is seen in the other 
genera, which, according to Simpson, have no such sulci. However, when the 
water-tubes are very large, as for instance in Lampsilis alata and gracilis, the 
sulci are a little more distinct on the lateral faces. 
In the genus Ptychobranchus (Pl. LX XXVIII, figs. 13, 14), the bulging out 
of the edge takes place all along the gill, which, in addition, is peculiarly folded, 
as is well known. In the rest of the genera the bulging is localized, and it is always 
the posterior section of the outer gill which shows this structure, sometimes to a 
greater, sometimes to a lesser extent. The fact that the marsupium is restricted 
in these genera, is a character emphasized by Simpson. There is some differentia- 
